Use of dialkylene glycol ester to reduce friction in vehicles with hybrid engines

ABSTRACT

The present invention relates to the use of a diester of formula (I) in a lubricant composition for a vehicle equipped with a hybrid engine, to reduce friction when the engine is used at low speed and/or when cold.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a U.S. National Phase Application under 35U.S.C. § 371 of International Patent Application No. PCT/EP2021/078956filed Oct. 19, 2021, which claims priority of French Patent ApplicationNo. 20 10736 filed Oct. 20, 2020. The entire contents of which arehereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to the use of dialkylene glycol ester in alubricant composition for vehicles equipped with a hybrid engine, toreduce the coefficient of friction when using the engine at low speedand/or when cold.

BACKGROUND

Improving the performance of lubricants is a constant area of interest.In particular, to meet increasing environmental requirements, it isincreasingly sought to improve the performance of lubricants and inparticular those for new generation vehicles, which include hybridvehicles.

However, few compositions have a particular focus on use thereof in ahybrid vehicle. Hybrid vehicles have particular constraints and inparticular when used at low speed or when cold i.e. over specificperiods which could be termed «start of cycle» periods, after vehiclestart-up. Over these cycles, the temperatures of the oils used in ahybrid vehicle are low, lower than the temperatures in a vehicle with aninternal combustion engine, and it would be an essential advantage topropose a formulation exhibiting better performance during theseparticular periods of use.

Against all expectations, the inventors have found that it is possibleto gain access to lubricant compositions having increased efficiency inFuel-Eco terms for cycles of use called low speed or cold cycles.

SUMMARY

It is therefore one object to the present invention to propose anadditive for a lubricant composition for a vehicle equipped with ahybrid engine, to reduce the coefficient of friction when the engine isused at low speed.

A further object of the invention is also to propose an additive for alubricant composition for a vehicle equipped with a hybrid engine, toreduce the coefficient of friction during cold cycle use of the engine.

Other objects will become apparent on reading the following descriptionof the invention.

DETAILED DESCRIPTION

These objects are achieved with the present invention which relates tothe use of a diester of formula (I) in a lubricant composition forvehicles equipped with a hybrid engine, to reduce the coefficient offriction when the engine is used at low speed and/or when cold.

R^(a)—C(O)—O—([C(R)₂]_(n)—O)_(s—C(O)—R) ^(b)   (1)

-   -   where:    -   R, each independently, are a hydrogen atom or a linear or        branched alkyl group having 1 to 5 carbon atoms, in particular a        methyl, ethyl or propyl group, preferably methyl;    -   s is 1 or 2;    -   n is 1, 2 or 3, on the understanding that when s differs from 1,        the n values can be the same or different;    -   R^(a) and R^(b), the same or different, are each independently        linear or branched, saturated or unsaturated hydrocarbon groups,        having a linear sequence of 6 to 18 carbon atoms;    -   provided that, when s is 2, and n all the same are 2, at least        one of the R groups is a linear or branched alkyl group having 1        to 5 carbon atoms; and provided that when s is 1 and n is 3, at        least one of the R groups linked to the carbon at beta position        of the oxygen atoms of the ester functions is a hydrogen atom.

The present invention also relates to a method for reducing thecoefficient of friction when a hybrid engine is used at low speed and/orwhen cold , comprising the lubricating of said engine with a lubricantcomposition comprising at least one base oil and a diester of formula(I)

R^(a)—C(O)—O—([C(R)₂]_(n)—O)_(s)—C(O)—R^(b)   (I)

-   -   where:    -   R, each independently, are a hydrogen atom or a linear or        branched alkyl group having 1 to 5 carbon atoms, in particular a        methyl, ethyl or propyl group, preferably methyl;    -   s is 1 or 2;    -   n is 1, 2 or 3, on the understanding that when s differs from 1,        the n values can be the same or different;    -   R^(a) and R^(b), the same or different, are each independently        linear or branched, saturated or unsaturated hydrocarbon groups        having a linear sequence of 6 to 18 carbon atoms;    -   provided that when s is 2 and n all the same are 2, at least one        of the R groups is a linear or branched alkyl group having 1 to        5 carbon atoms; and    -   provided that when s is 1 and n is 3, at least one of the R        groups linked to the carbon at beta position of the oxygen atoms        of the ester functions is a hydrogen atom.

In the present invention, by «low speed» it is meant a specific enginespeed. Engine speed designates the speed of rotation of the engine andis expressed in revolutions per minute. Low speed according to theinvention is characterized by a number of revolutions per minute (rpm)of between 600 and 2000 rpm, preferably between 900 and 1500 rpm.

In the present invention, by «when cold» it is meant use of the enginesat start-up and during phase 1 of the WLTC cycle, at which time theengine is at a temperature lower than 50° C., preferably between 20 and50° C., preferably between 25 and 40° C.

The reduction in the coefficient of friction is to be construed as areduction in comparison with the observation which would be made with alubricant composition not comprising the diester of the invention.

Preferably, in the diester of formula (I) of the invention, when sdiffers from 1, all the n values are the same.

Preferably, in the diester of formula (I) of the invention, n is 2 or 3,preferably 2.

Preferably, in the diester of formula (I) of the invention, at least oneof the R groups is a linear or branched alkyl group having 1 to 5 carbonatoms, preferably 1 to 4 carbon atoms, and more preferably is methyl,ethyl or propyl, advantageously methyl.

In one particular embodiment, the diester of formula (I) of theinvention is a diester of following formula (I′) :

R^(a)—C(O)—O—([C(R)₂]_(n)—O)—([C(R′)₂]_(m)—O)_(s-1)—C(O)—R^(b)   (I′)

-   -   where    -   R and R′, each independently, are a hydrogen atom or a linear or        branched alkyl group having 1 to 5 carbon atoms, preferably a        methyl, ethyl or propyl group, preferably a methyl group;    -   s is 1 or 2;    -   n is 2;    -   m is 2;    -   Ra and Rb, the same or different, are each independently linear        or branched, saturated or unsaturated hydrocarbon groups having        a linear sequence of 6 to 18 carbon atoms;    -   provided that when s is 2, at least one of the groups R or R′ is        a linear or branched alkyl group having 1 to 5 carbon atoms.

Advantageously, at least one of the groups R or R′ in the diester offormula (I′) is a linear or branched alkyl group having 1 to 5 carbonatoms, preferably 1 to 4 carbon atoms, preferably methyl, ethyl orpropyl, advantageously methyl.

Preferably, in the compounds of formula (I) and (I′), Ra and Rb have alinear sequence of 7 to 14 carbon atoms, preferably 8 to 12 carbonatoms, more preferably 8 to 11 carbon atoms, in particular 9 or 11carbon atoms.

In one particular embodiment, in formulas (I) or (I′), s is 2.

The diester of the invention is preferably a compound of formula (I′a):

R^(a)—C(O)—O—([C(R)₂]_(n)—O)—([C(R′)₂]_(m)—O)—C(O)—R^(b)   (I′a)

-   -   where:        -   R and R′ are each independently a hydrogen atom or a linear            or branched alkyl group having 1 to 5 carbon atoms,            preferably methyl, ethyl or propyl, preferably methyl;        -   n is 2;        -   m is 2;        -   Ra and Rb, the same or different, are each independently            linear or branched, saturated or unsaturated hydrocarbon            groups having a linear sequence of 6 to 18 carbon atoms;    -   provided that at least one of the groups R or R′ is a linear or        branched alkyl group having 1 to 5 carbon atoms, preferably        methyl, ethyl or propyl, preferably methyl.

Preferably, at least one of the R groups is a linear or branched alkylgroup having 1 to 5 carbon atoms, preferably methyl, ethyl or propyl,preferably methyl; and at least one of the R′ groups is a linear orbranched alkyl group having 1 to 5 carbon atoms, preferably methyl,ethyl or propyl, preferably methyl.

Further preferably, in the diester of formula (I′a) one of the R groupsis a linear or branched alkyl group having 1 to 5 carbon atoms,preferably methyl, ethyl or propyl, preferably methyl; and one of the R′groups is a linear or branched alkyl group having 1 to 5 carbon atoms,preferably methyl, ethyl or propyl, preferably methyl; the other R andR′ groups representing hydrogen atoms.

In one particular embodiment, the diester of the invention is a compoundof formula (I″a)

R^(a)—C(O)—O—CHR¹—CHR²—O—CHR³—CHR⁴—O—C(O)—R^(b)   (I″a)

-   -   where:        -   one of the groups R¹ and R² is a linear or branched alkyl            group having 1 to 5 carbon atoms, the other being a hydrogen            atom;        -   one of the groups R³ and R⁴ is a linear or branched alkyl            group having 1 to 5 carbon atoms, the other being a hydrogen            atom;        -   Ra and Rb, the same or different, are such as previously            defined.

Preferably, in the compounds of formula (I″a):

-   -   one of the groups R¹ and R² is a methyl, ethyl or propyl group,        preferably methyl, the other being a hydrogen atom;    -   one of the groups R³ and R⁴ is a methyl, ethyl or propyl group,        preferably methyl, the other being a hydrogen atom.

In another alternative embodiment, in the compounds of formula (I) or(I′), s is 1 and the diesters of the invention have the formula (I′b):

R^(a)—C(O)—O—([C(R)₂]_(n)—O)—C(O)—^(b)   (I′b)

-   -   where:        -   R, each independently, are a hydrogen atom or a linear or            branched alkyl group having 1 to 5 carbon atoms, preferably            methyl, ethyl or propyl, preferably methyl;    -   n is 2;    -   R^(a) and R^(b), the same or different, are each independently        linear or branched, saturated or unsaturated hydrocarbon groups        having a linear sequence of 6 to 18 carbon atoms.

Preferably, in formula (I′b), at least one of the R groups is a linearor branched alkyl group having 1 to 5 carbon atoms, preferably methyl,ethyl or propyl, preferably methyl.

Preferably, in formula (I′b), one of the R groups is a linear orbranched alkyl group having 1 to 5 carbon atoms, preferably methyl,ethyl or propyl, preferably methyl, the others being hydrogen atoms.

In the present invention, it is to be understood that the expression«having x to y carbon atoms» also includes the limits x and y.

In the present invention, it is to be understood that by «linearsequence of x to y carbon atoms» it is meant a saturated or unsaturatedcarbon chain, preferably saturated, having x to y carbon atoms followingafter each other, any carbon atoms which may be present at the branchesof the carbon chain not being taken into account in the number of carbonatoms (x-y) forming the linear sequence.

In one particular embodiment, in formulas (I), (I′), (I′a), (I″a) or(I′b), Ra and Rb the same or different are of plant, animal or petroleumorigin.

In one particular embodiment, in formulas (I), (I′), (I′a), (I″a) or(I′b), Ra and Rb the same or different are saturated groups.

In one particular embodiment, in formulas (I), (I′), (I′a), (I″a) or(I′b), Ra and Rb the same or different are linear groups. In particular,Ra and Rb the same or different are saturated linear hydrocarbon groupshaving 6 to 18 carbon atoms, preferably 7 to 17 carbon atoms, morepreferably 7 to 14 carbon atoms, further preferably 8 to 12 carbon atomsand in particular 9 or 12 carbon atoms.

In another preferred embodiment, in formulas (I), (I′), (I′a), (I″a) or(I′b), Ra and Rb the same or different are saturated linear alkyl groupshaving 6 to 18 carbon atoms, preferably 7 to 17 carbon atoms, morepreferably 7 to 14 carbon atoms, further preferably 8 to 12 carbon atomsand in particular 9 or 12 carbon atoms.

Preferably, Ra and Rb are the same.

The diesters of formula (I) can be commercially available diesters orthey can be prepared following the synthesis methods described in theliterature and known to persons skilled in the art, in particularaccording to the methods described in WO201925446.

Preferably, the diester of formula (I) is added in a proportion of 1 to40% by weight relative to the total weight of the lubricant composition,preferably 10 to 30 %.

The base oils used in the lubricant compositions of the invention can beoils of mineral or synthetic origin belonging to Groups I to V in theclasses defined by the API classification (or the equivalents thereof inthe ATIEL classification (Table 1), or mixtures thereof.

TABLE 1 Content of Viscosity saturated Index substances Sulfur content(VI) Group I  <90% >0.03% 80 ≤ VI < 120 Mineral oils Group II ≥90%≤0.03% 80 ≤ VI < 120 Hydrocracked oils Group III ≥90% ≤0.03% ≥120Hydro-isomerized oils Group IV Polyalphaolefins (PAO) Group V Esters andother bases not included in Groups I to IV

The mineral base oils of the invention include any type of base oilobtained by atmospheric and vacuum distillation of crude petroleum,followed by refining operations such as solvent extraction,deasphalting, solvent dewaxing, hydrotreatment, hydrocracking,hydroisomerization and hydrofinishing.

Mixtures of synthetic and mineral oils can also be used.

The base oils of the lubricant composition of the invention can also beselected from among synthetic oils such as some esters of carboxylicacids and alcohols, and polyalphaolefins. The polyalphaolefins used asbase oil are obtained for example from monomers having 4 to 32 carbonatoms, for example from octene or decene, and in which viscosity at 100°C. is between 1.5 and 15 mm2.s-1 according to standard ASTM D445. Themolecular weight average thereof is generally between 250 et 3000according to standard ASTM D5296.

The lubricant composition of the invention may comprise at least 50% byweight of base oil relative to the total weight of the composition. Moreadvantageously, the lubricant composition of the invention comprises atleast 60% by weight, or even at least 70% by weight of base oilsrelative to the total weight of the lubricant composition. Morepreferably, the lubricant composition of the invention comprises from 75to 97% by weight of base oils relative to the total weight of thecomposition.

The composition of the invention may also comprise at least oneadditive.

Numerous additives can be used in the lubricant compositions of theinvention.

The preferred additives for the lubricant composition of the inventionare selected from among detergent additives, friction modifyingadditives differing from the molybdenum compounds defined above, extremepressure additives, dispersants, pour point activators, defoamingagents, thickeners, and mixtures thereof.

Preferably, the lubricant compositions of the invention comprise atleast one extreme pressure additive, or a mixture.

Antiwear additives and extreme pressure additives protect againstsurface frictions by forming a protective film adsorbed on surfaces.

There exists a wide variety of antiwear additives. Preferably, for thelubricant compositions of the invention, the antiwear additives areselected from among additives comprising phosphorus and sulfur such asalkylthiophosphate metals, in particular zinc alkylthiophosphate, andmore specifically zinc dialkyldithiophosphate or ZnDTP. The preferredcompounds have the formula Zn((SP(S)(OR)(OR′))2, where R and R′, thesame or different, are each independently an alkyl group, preferably analkyl group having 1 to 18 carbon atoms.

Amine phosphates are also antiwear additives that can be used in thelubricant compositions of the invention. However, the phosphorus atomscontributed by these additives may act as poison for the catalyticsystems of motor vehicles since they generate ash. It is possible tominimise these effects by substituting part of the amine phosphates byadditives not contributing phosphorus such as polysulfides for example,in particular sulfur- containing olefins.

Advantageously, the lubricant compositions of the invention may comprisefrom 0.01 to 6 % by weight, preferably from 0.05 to 4% by weight, morepreferably from 0.1 to 2% by weight of antiwear and extreme pressureadditives, relative to the total weight of the lubricant composition.

Advantageously, the lubricant compositions of the invention comprisefrom 0.01 to 6 by weight, preferably from 0.05 to 4% by weight, morepreferably from 0.1 to 2% by weight of antiwear additives (or anti-wearcompound) relative to the total weight of the lubricant composition.

Advantageously, the compositions of the invention may comprise at leastfriction modifying additive differing from the molybdenum compounds ofthe invention. The friction modifying additives can be selected inparticular from among compounds contributing metal elements and ash-freecompounds. Among those compounds contributing metal elements mention canbe made of complexes of transition metals such as Mo, Sb, Sn, Fe, Cu, Znin which the ligands can be hydrocarbon compounds comprising atoms ofoxygen, nitrogen, sulfur or phosphorus. The ash-free friction modifyingadditives are generally of organic origin and can be selected from amongthe monoesters of fatty acids and polyols, alkoxylated amines,alkoxylated fatty amines, fatty epoxides, fatty epoxide borates, fattyamines or the esters of glycerol acid. In the invention, the fattycompounds comprising at least one hydrocarbon group have 10 to 24 carbonatoms.

Advantageously, the lubricant composition of the invention may comprisefrom 0.01 to 2% by weight, or from 0.01 to 5% by weight, preferably from0.1 to 1.5% by weight or from 0.1 to 2% by weight of friction modifyingadditives differing from the molybdenum compounds of the invention,relative to the total weight of the lubricant composition.

Advantageously, the lubricant composition of the invention may compriseat least one antioxidant additive.

Antioxidant additives generally delay degradation of the lubricantcomposition. This degradation most often translates as the formation ofa deposit, the presence of sludge or by an increase in the viscosity ofthe lubricant composition.

Antioxidant additives generally act as radical scavengers orhydroperoxide decomposers. Among the antioxidants routinely used,mention can be made of antioxidants of phenolic type, antioxidants ofamine type, antioxidants containing sulfur and phosphorus. Some of theseantioxidants, for example those comprising sulfur and phosphorus, maygenerate ash. Phenolic antioxidant additives can be ash-free or they canbe in the form of neutral or basic salts. The antioxidant additives canparticularly be selected from among sterically hindered phenols,sterically hindered phenol esters, sterically hindered phenolscomprising a thioether bridge, diphenylamines, diphenylaminessubstituted by at least one C₁ to C₁₂ alkyl group,N,N′-dialkyl-aryl-diamines, and mixtures thereof.

Preferably, in the invention, the sterically hindered phenols areselected from among compounds comprising a phenol group in which atleast one of the carbon atoms in the vicinity of the carbon atomcarrying the alcohol function is substituted by at least one C₁ to C₁₀alkyl group, preferably a C₁ to C₆ alkyl group, preferably a C₄ alkylgroup, preferably a tert-butyl group.

Amine compounds are another class of antioxidant additives which can beused, optionally in combination with phenolic antioxidant additives.Examples of amine compounds are aromatic amines, for example aromaticamines having the formula NRaRbRc where Ra is an optionally substitutedaliphatic group or aromatic group, Rb is an optionally substitutedaromatic group, Rc is a hydrogen atom, an alkyl group, an aryl group ora group of formula RdS(O)zRe where Rd is an alkylene or alkenylenegroup, Re is an alkyl group, an alkenyl group or aryl group and z is 0,1 or 2.

Alkyl-phenols containing sulfur or the alkali or alkaline-earth metalsalts thereof can also be used as antioxidant additives.

Other classes of antioxidant additives are copper-containing compounds,for example copper thio- ou dithio-phosphate, salts of copper andcarboxylic acids, dithiocarbamates, sulfonates, phenates, copperacetylacetonates. The salts of copper I and II, the salts of succinicacid or anhydride can also be used.

The lubricant compositions of the invention may also comprise any typeof antioxidant known to persons skilled in the art.

Advantageously, the lubricant composition comprises at least oneash-free antioxidant additive.

Also advantageously, the lubricant composition of the inventioncomprises from 0.1 to 2% by weight of at least one antioxidant additive,relative to the total weight of the composition.

The lubricant composition of the invention may also comprise at leastone detergent additive.

Detergent additives generally allow a reduction in the formation ofdeposits on the surface of metal parts, by dissolving secondary productsof oxidation and combustion.

The detergent additives able to be used in the lubricant compositions ofthe invention are generally known to persons skilled in the art. Thedetergent additives can be anionic compounds comprising a longlipophilic hydrocarbon chain and hydrophobic head. The associated cationcan be a metal cation of an alkali or alkaline-earth metal.

The detergent additives are preferably selected from among the alkali oralkaline-earth metal salts of carboxylic acids, sulfonates, salicylates,naphthenates, and phenate salts. The alkali and alkaline-earth metalsare preferably calcium, magnesium, sodium or barium.

These metal salts generally comprise the metal in stoichiometric amountor in excess, i.e. in an amount greater than the stoichiometric amount.They are then overbased detergents; the excess metal imparting theoverbased nature to the detergent additive is generally in the form ofan oil-insoluble metal, for example carbonate, hydroxide, oxalate,acetate, glutamate, preferably carbonate.

Advantageously, the lubricant composition of the invention may comprisefrom 0.5 to 8%, or from 2 to 4% by weight of overbased detergentadditive relative to the total weight of the lubricant composition.

Also advantageously, the lubricant composition of the invention may alsocomprise a pour point depressant additive.

By slowing the formation of wax crystals, the pour point depressantadditive generally improves the cold-start properties of the lubricantcomposition of the invention.

As examples of pour point depressant additives, mention can be made ofalkyl polymethacrylates, polyacrylates, polyarylamides,polyalkylphenols, polyalkylnaphthalene, alkyl polystyrenes.

Advantageously, the lubricant composition of the invention may alsocomprise a dispersant.

Dispersants can be selected from among Mannich bases, succinimides, andderivatives thereof.

Also advantageously, the lubricant composition of the invention maycomprise from to 10% by weight of dispersant relative to the totalweight of the lubricant composition.

Advantageously, the lubricant composition of the invention may furthercomprise at least one other additional polymer improving the viscosityindex. As examples of additional polymer improving the viscosity index,mention can be made of polymeric esters, the homopolymers or copolymers,whether or not hydrogenated, of styrene, butadiene, and isoprene,polymethacrylates (PMA). Also advantageously, the lubricant compositionof the invention may comprise from 1 to 15% by weight of additiveimproving the viscosity index, relative to the total weight of thelubricant composition.

The lubricant composition of the invention may also comprise at leastone thickening agent.

The lubricant composition of the invention may also comprise a defoamingagent or demulsifying agent.

Preferably, the lubricant composition of the invention further comprisesat least one antiwear agent, in particular zinc-based, and particularlyZnDTP.

In the present invention, by hybrid motor vehicle or vehicles equippedwith a hybrid engine, it is meant vehicles having recourse to twoseparate sources of energy for powering.

In particular, hybrid vehicles associate an internal combustion enginewith an electric motor, said electric motor taking part in vehicletraction.

The invention also relates to a method for reducing friction when ahybrid engine is used at low speed and/or when cold, which comprises theaddition of a diester of the invention to the lubricant composition ofsaid engine.

The invention also relates to a method for reducing friction when ahybrid engine is used at low speed and/or when cold, which comprises thelubrication of said engine with a lubricant composition comprising adiester of the invention.

The present application is described below with the aid of nonlimitingexamples.

Example 1: Preparation of Lubricant Compositions

The following lubricant compositions (CL: compositions of the invention,and CC: comparative compositions) were prepared following methods knownto persons skilled in the art.

TABLE 2 CC1 CC2 CL1 CC3 CL2 (wt. %) (wt. %) (wt. %) (wt. %) (wt. %)Additive package 12.7 12.7 12.7 12.7 12.7 MoDTC 0.5 0.5 0.5 0.5 0.5 PPD0.2 0.2 0.2 0.2 0.2 Antioxidant 0.5 0.5 0.5 0.5 0.5 Viscosity 0.4 0.80.8 1.3 1.5 modifier PAO 0 30 0 0 0 Ester of 0 0 30 0 20 formula (I)Base oil 85.7 55.3 55.3 84.8 64.6 Grade 0W12 0W12 0W12 0W12 0W12

TABLE 3 CC4 CL3 CC5 CL4 CC6 (wt. %) (wt. %) (wt. %) (wt. %) (wt. %)Additive package 13 13 12.6 12.6 12.6 MoDTC 0.5 0.5 0.5 0.5 0.5 PPD 0.20.2 0.2 0.2 0.2 Viscosity 1.3 4 4 4 4 modifier PAO 0 0 0 0 20 Ester of 010 0 20 0 formula (I) Base oil 85 72.3 82.7 62.7 62.7 Grade 0W12 0W120W12 0W12 0W12

TABLE 4 CC7 CL5 CC8 (wt. %) (wt. %) (wt. %) Additive package 10.1 10.110.1 MoDTC 0.4 0.4 0.4 Viscosity modifier 6.7 6.7 5.8 Ester of formula(I) 0 20 0 Base oil 82.8 62.8 83.7 Grade 0W20 0W20 0W20

Example 2: Results of FMEP Tests

A FMEP test bed (Friction Mean Effective Pressure) was used to measureengine friction. The engine was driven by a generator and the torque tobe supplied by this generator was measured (this torque is therefore theimage of engine friction).

In this type of test, the engine does not therefore burn any fuel, it isjust driven in exactly the same manner as in a vehicle engine brakingsituation.

The properties of the test bed in terms of engine speed and temperaturewere the following:

-   -   Speed: 900*-6000 rpm    -   Oil temperature: 35-110° C.**    -   Water temperature: 37-90° C.**

4 thermal conditions were tested:

-   -   Engine oil temperature=37° C./Temperature of coolant=37° C.    -   Engine oil temperature=50° C./Temperature of coolant=50° C.    -   Engine oil temperature=80° C./Temperature of coolant=80° C.    -   Engine oil temperature=110° C./Temperature of coolant=90° C.

The sequences of tested engine speeds were the following:

900, 1450, 2000, 2500, 3000, 3500, 4000, 4500

For the test, complete mapping of engine friction was determinedaccording to the different rotation speeds and different oil and watertemperatures.

The friction mapping obtained at the end of the test was composed of 8engine speeds and 4 temperatures.

The time required for this mapping was about 11 h.

The results are expressed as a gain in mean coefficient of friction.

This mapping was then compared with that of a so-called reference oilwhich was measured before and after the testing of the candidate oil todetermine any drift from the test mean and integrate the same in theinterpretation of results.

The composition of the so-called reference oil was as follows:

TABLE 5 PAO 10.00 PAO 10.00 MoDTC 0.50 Pour point depressant 0.20 Baseoil 61.70 Additive package 12.70 Amine antioxidant 0.50 Polymerimproving the viscosity index 4.40

It is with reference to this oil that the percentages for gain infriction were calculated.

The results in terms of gain in friction compared with the reference oilare given in Tables 6 to 8 below.

TABLE 6 CC1 (%) CC2 (%) CL1 (%) CC3 (%) CL2 (%) 30° C. 1.42 1.06 3.381.20 3.35 50° C. 1.7 1.71 2.48 1.85 2.72

TABLE 7 CC4 (%) CL3 (%) CC5 (%) CL4 (%) CC6 (%) 30° C. 1.44 3.55 3.014.19 2.66 50° C. 1.27 2.93 2.67 2.88 2.25

TABLE 8 CC7 (%) CL5 (%) CC8 (%) 30° C. 3.89 6.25 0.2 50° C. 2.78 3.980.54

The results show the capability of the ester of the invention to improvegains in friction over temperatures of between 30° C. and 50° C.

1-7. (canceled)
 8. A method for reducing friction when using a hybridengine at low speed and/or when cold comprising the addition, to thelubricant composition of said engine, of a diester of formula (I)R^(a)—C(O)—O—([C(R)₂]_(a)—O)_(b)—C(O)—R^(b)   (I) where: R, eachindependently, are a hydrogen atom or a linear or branched alkyl grouphaving 1 to 5 carbon atoms; s is 1 or 2; n is 1, 2 or 3, on theunderstanding that when s differs from 1, the n values can be the sameor different; R^(a) and R^(b), the same or different, are eachindependently linear or branched, saturated or unsaturated hydrocarbongroups, having a linear sequence of 6 to 18 carbon atoms; provided thatwhen s is 2 and n all the same are 2, at least one of the R groups is alinear or branched alkyl group having 1 to 5 carbon atoms; and providedthat when s is 1 and n is 3, at least one of the R groups linked to thecarbon at beta position of the oxygen atoms of the ester functions, is ahydrogen atom.
 9. The method according to claim 8, wherein the diesteris a diester of formula (I), where when s differs from 1, all the nvalues are the same.
 10. The method according to claim 8, wherein thediester is a diester of formula (I) where n is 2 or
 3. 11. The methodaccording to claim 8, wherein the diester is a diester of formula (I)where at least one of the R groups is a linear or branched alkyl grouphaving 1 to 5 carbon atoms.
 12. The method according to claim 8, whereinthe diester of formula (I) is a diester of following formula (I′):R^(a)—C(O)—O—([C(R)₂]_(n)—O)—([C(R′)₂]_(m)—O)_(b-1)—C(O)—R^(b)   (I′)where: R and R′, each independently, are a hydrogen atom or a linear orbranched alkyl group having 1 to 5 carbon atoms; s is 1 or 2; n is 2; mis 2; R^(a) and R^(b), the same or different, are each independentlylinear or branched, saturated or unsaturated hydrocarbon groups having alinear sequence of 6 to 18 carbon atoms; provided that when s is 2, atleast one of the groups R or R′ is a linear or branched alkyl grouphaving 1 to 5 carbon atoms.
 13. The method to claim 8, wherein thediester is a compound of formula (I″a)R^(a)—C(O)—O—CHR¹—CHR²—O—CHR³—CHR⁴—O—C(O)—R^(b)   (I″a) where: one ofthe groups R¹ and R² is a linear or branched alkyl group having 1 to 5carbon atoms, the other being a hydrogen atom; one of the groups R³ andR⁴ is a linear or branched alkyl group having 1 to 5 carbon atoms, theother being a hydrogen atom, R^(a) and R^(b), the same or different, aresuch as defined in claim
 8. 14. The method according to claim 8, whereinthe diester of formula (I) is added in a proportion of 1 to 40% byweight relative to the total weight of the lubricant composition.